Wireless local area networks (WLANs) as defined e.g. in the IEEE 802.11 specifications are almost omnipresent today. The increase of throughput of the available channel was one major issue, and research has been focused on improving the modulation and coding within the Physical Layer. By employing orthogonal frequency division multiplexing (OFDM) in combination with high-rate signal constellations, up to 54 Mbit/s could be achieved. This huge performance jump—even if achieved only for very limited distances—is caused by to inherent features of OFDM, which have become especially attractive for high bit-rate systems. In OFDM, the given system bandwidth is split into many sub-channels, also referred to as sub-carriers. Instead of transmitting symbols sequentially through one (very broad) channel, multiple symbols are transmitted in parallel. This leads to much longer symbol durations, such that the impact of inter-symbol interference can be reduced significantly, so that no additional measures like costly equalization are necessary.
The 802.11 standard makes it mandatory that all stations implement a distributed coordination function (DCF) which is a form of carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CA is a contention-based protocol making certain that all stations first sense the medium before transmitting. The main goal is to avoid having stations transmit at the same time, which results in collisions and corresponding retransmissions. If a station wanting to send a frame senses energy above a specific threshold on the medium (which could mean the transmission of another station), the station wanting access will wait until the medium is idle before transmitting the frame. The collision avoidance aspect of the protocol pertains to the use of acknowledgements that a receiving station sends to the sending station to verify error-free reception. Although somewhat more complex, this process of accessing the medium can be seen as a meeting where everyone is polite and each person only speaks when no one else is talking. In addition, participants who understand what the person is saying nod their head in agreement.
Because of its nature, DCF supports the transmission of asynchronous signals. A distinguishing factor of asynchronous signaling is that there are no timing requirements between data carrying frames. For example, the DCF protocol doesn't make any attempt to deliver a series of data frames within any timeframe or at any instant in time. As a result, there is a random amount of delay between each data frame transmission. This form of synchronization is effective for network applications, such as e-mail, Web browsing and VPN access to corporate applications.
A potential for further bit-rate increases is seen in a use of multiple-input multiple-output (MIMO) antenna systems. Hence, a new Medium Access Control (MAC) protocol mechanism has been proposed, which supports multi-user (MU) MIMO transmissions in WLANs according to IEEE 802.11 based standards. The proposed new protocol extends the DCF with single-user (SU) MIMO in such a way that different stations can be destination stations for packets inside a MIMO frame (which is a set of packets transmitted simultaneously on different spatial streams).
According to a randomly operating backoff procedure, a station with a data packet to transmit generates a random number between 0 and a contention window (CW) size, which determines the duration of the backoff timer as counted in number of timeslots. The CW has a minimum starting value of 15, doubles after a collision, can rise up to 1023, and is decremented after a successful transfer, indicated by an acknowledgement (ACK) frame. After detecting the medium free for the duration of DCF Inter-Frame Space (DIFS), the mobile station counts down the backoff timer until it reaches zero and then starts its transmission. If during the countdown another mobile station occupies the medium, all mobile stations in backoff interrupt their count down and defer until they detect the medium free for at least DIFS. The standard includes an optional Request-to-Send (RTS)—Clear-to-Send (CTS) handshake prior to the transmission.
In an association procedure prior to data transmissions, stations share among each other the information about their hardware capabilities. Information concerning used antenna elements can be exchanged using an extended form of RTS and CTS control frames, described in the following paragraphs.
The extended RTS frame—MIMO-RTS (M-RTS) and the extended CTS frame—MIMO-CTS (M-CTS) can be based on the structure of the IEEE 802.11a standard RTS and CTS frames. In order to support multiple antennas, both have a new field, e.g. a bitmap, where each bit stands for one antenna. A bitmap of a length of one byte can thus support up to eight antennas. Of course, the bitmap field can be longer or shorter depending on the number of antennas supported by the mobile stations of a given system. In the M-RTS frame this field may be called Proposed Antenna Bitmap (PAB) and may encode the chosen subset of available antennas proposed for the following transmission. The receiver of the frame confirms which antennas should be active in a Confirmed Antenna Bitmap (CAB) field of the M-CTS frame. The ACK frame is also extended to support per-stream acknowledgements. More specifically, the MIMO-ACK (M-ACK) frame may have a one byte long bitmap field called Acknowledged Packet Bitmap (APB) to confirm the reception of each packet from different streams separately. It contains positive and negative acknowledgements for each spatial stream. It can still be immediately acknowledging, although there are multiple packets being transmitted at a time. The length of the bitmaps (L) can be arbitrary.
The following points give an overview of the additional MAC protocol functionality of M-DCF during a transmission cycle, omitting the ones concerning CSMA/CA:                The transmitter sends an M-RTS frame, setting binary “1”s in the PAB field for available antennas for the next transmission.        Immediately afterwards, when the receivers have already read the M-RTS frame and available antennas, the transmitter sends a training sequence for each available antenna for channel estimation. Alternatively, channel estimation is done in parallel to M-RTS frame.        The receiver estimates the channel, and responds with an M-CTS frame, setting binary “1”s in the CAB field for the antennas which should be used for the transmission. The MIMO scheme can be selected based on at least one of the stations' hardware capabilities, quality of service (QoS) demands of the connection, radio propagation conditions, and current status of the network. How the receiver chooses the antennas can be its internal procedure. This procedure of choosing the applied MIMO scheme during network operation of per frame basis provides fast link adaptation.        After reception of the M-CTS frame, the transmitter transmits (one or more) packets based on the receiver's instructions about the antennas to be used, each using a separate antenna.        After the reception of the data frame(s), the receiver checks the correctness of the received packets, and may create an extended M-ACK frame to inform the transmitter about the outcome of the transmission. Binary “1”s are set in the M-ACK bitmap for the correctly received packets.        When the transmitter receives the M-ACK frame, it removes the packets from the queue and initiates another transmission. If the M-ACK frame is lost, or if it has never been transmitted at all, after a timeout the transmitter will retransmit the data.        
An M_DCF protocol which is restricted to carrying a MAC packet in one spatial stream performs very well in networks of users with heavy load, because of its increased system capacity achieved by using MIMO technology. However, when the load is not high, the packet delay grows due to the fact that according to the protocol, a station does not start a transmission before having a number n of packets to transmit, where n is the number of spatial streams. If the mean inter-arrival time between two packets for a connection is T, maximum allowed delay should be higher then (n−1)T plus the average transmission window length (including accessing the channel). Otherwise some packets will be discarded at the transmitter because of exceeded delay. This relation gives the lower bound for the offered load under which the delay requirement can still be fulfilled. Increasing the offered load (up to the point when the network capacity is reached) will improve the delay characteristic.
In ubiquitous networking, a station might be communicating with multiple other users at a time. Applying M-DCF directly would lead to high delays for each connection, although the station would actually have enough packets to build a MIMO frame. Therefore, combining the traffic belonging to multiple users should be enabled. This transmission strategy will immediately improve the delay characteristic, because the traffic received from all combined connections will contribute to the building of MIMO frames. In the previous lower bound calculation, the parameter T now corresponds to the inter-arrival time between any two packets, independent of the destination, and therefore the lower bound for the offered load of each individual connection is lower. Under heavy load, MU transmissions are means of reducing jitter. Moreover, Improving the delay characteristic is especially important for applications such as Voice-over-IP (VoIP), Video Conferencing, interactive gaming, etc.
J. Gross, et al., “802.11 DYN: Protocol Extension for the Application of Dynamic OFDM(A) Schemes in 802.11a/g Systems”, Telecommunication Networks Group (TKN) Technical Report TKN-07-002 describes a proposal how the widely accepted IEEE 802.11a/g systems might be extended to support dynamic OFDM while assuring backward compatibility. A set of protocol modifications is presented, which support dynamic OFDM schemes both for point-to-point (e.g. uplink) and point-to-multi-point (e.g. downlink) transmission scenarios. The proposed RTS frame corresponds to the regular RTS frame (namely it contains only one transmit address and one receive address) with a new Physical Layer Convergence Protocol (PLCP) header to which a list of multiple receiver addresses is added.
However, legacy devices according to older or former standards in the network won't be able to decode the signal. Namely, the legacy devices may not be able to determine or extract the correct bits out of the signal. This means that the legacy devices wouldn't learn about the duration of the intended transmission because this information is contained in the RTS frame. Therefore, the proposed RTS transmission cannot be regarded as a broadcast transmission, because it cannot be understood (on the physical layer) by all stations. For this reason, the above prior art suggests transmitting, prior to RTS transmission, a CTS addressed to itself in the legacy physical layer, so that other legacy devices can decode the transmission and the MAC frame and set their network allocation vectors (NAV) for their transmission timing appropriately.
Additionally, in the above prior art, identification of the polled stations may be based on a 4-bit identification, for example. Yet, the identification is used for MAC purposes, namely, a CTS frame must be constructed and transmitted. This means, after receiving the RTS frame, the PHY layer extracts the identification list, and the MAC layer must then check whether the list contains an identification for itself, decode the frame, possibly overwrite the receiver address (which is the address of one of the addressees), then construct the CTS frame, and then take turn in sending the RTS. This requires substantial modifications of the standard RTS/CTS procedure and has significant impact on the architecture of the receiver, e.g., new information passing between PHY and MAC must be defined. Furthermore, some specific conditions must be applied in the interpretation and processing of the RTS frame.
As the above prior art also requires an assignment of this 4-bit identifications to the stations. This may be done during the association by the access point (AP) and may imply that an AP can only be associated with 16 stations having this MU-OFDM capability at one particular time.